1. Field of the Invention
The present invention relates to a charge transport material that can be applied to organic electronic devices such as electrophotographic photoreceptors, organic electroluminescent elements, photorefractive elements, electrochromic elements, photosensors, and solar cells. More particularly, the present invention relates to a charge transport material having excellent charge transport capability and a process for producing the same.
2. Background Art
Charge transport materials are materials having such a charge transport capability that, upon charge injection, charge is diffused and moved, for example, due to charge concentration gradient and electric field gradient. A material which functions to transport electron as the charge is called an electron transport material, and a material which functions to transport hole as the charge is called a hole transport material. The electron transport material and the hole transport material are collectively called a charge transport material. The charge transport material has been extensively studied as a material indispensable for the preparation of organic electronic devices such as electrophotographic photoreceptors, organic electroluminescent elements, photorefractive elements, electrochromic elements, photosensors, and solar cells.
Basic properties required of the charge transport material include charge acceptability for electron or hole or both electron and hole in a neutral state, a high charge transport capability, good film formability, and a stable amorphous state of the film.
The charge transport material is in many cases used as an even thin film. Therefore, it is important that a film be easily formed from the charge transport material. When a thin film having a thickness of not more than 1 μm is formed using a charge transport material of a low molecular compound, vacuum deposition is generally used for film formation. The vacuum deposition requires a larger deposition apparatus than coating methods, leading to an increase in cost. In the vacuum deposition, the adoption of a large-area substrate on which the charge transport layer is to be formed is difficult. Further, when the low molecular compound is used solely as the charge transport material, for example, the mechanical strength and heat stability of the formed thin film are unsatisfactory. For this reason, a method is also adopted in which a polymer is used as a binder and the low molecular compound is dispersed in a binder to prepare a coating liquid which is then coated to form a film.
Incidentally, most of charge transport materials are hole transport materials, and tertiary amine derivatives such as triarylamine have extensively been used as hole transport materials. On the other hand, for the electron transport material, for example, since the solubility thereof in organic solvents is not high, the electron transport material is not suitable for the formation of a film by coating. Further, the mobility of electron is not high. For the above reason, as compared with the hole transport material, the number of types of the electron transport material is smaller.
Some of charge transport materials are bipolar charge transport materials that have both a hole transport function and an electron transport function. An example of a bipolar charge transport material is CBP (4,4-bis(carbazol-9-yl)-biphenyl) (see, for example, Japanese Patent Laid-Open No. 168443/1998). This CBP has a bipolar property and, in addition, can form a highly transparent thin film and is highly compatible with a luminescent dopant. Therefore, CBP is suitable for use in the formation of a charge transport layer in organic electroluminescent elements and thus has been suitably used in organic electroluminescent elements using a phosphorescent dopant (M. A. Baldo et al., Nature, vol. 395, p. 151 (1998), M. A. Baldo et al., Applied Physics Letters, vol. 75, p. 4 (1999), M. A. Baldo et al, Nature, vol. 403, p. 750 (2000)).
In charge transport materials such as CBP, however, vacuum deposition has still mainly been used for film formation. Further, since CBP molecules are likely to take a planar structure, an amorphous film formed of CBP is likely to be crystallized with the elapse of time or upon heating. For this reason, the application of CBP to electronic devices involving generation of heat due to Joule's heat, particularly such as organic electroluminescent elements, requires doping of CBP with a large amount (about 5 to 10% by mass) of a dopant, or the use of a binder to prevent crystallization of CBP. Therefore, when CBP is solely used, a film cannot be formed by coating without difficulties.